CEE Jan-Mar 2012

diffusion path can only be shortened or the surtace of the fuel particles increased with increasingly finer grain sizes. For this reason , using a certain grinding technology, so far not yet adapted to be used on substitute fuels, the average grain size of dmax. 80 mm can be reduced 100 per cent to < 3mm. The operating principle of the mill is based on a rotor turning at high peripheral speed {approx. 90 to 100 m/s) and a high air flow rate, whereby the air stream in the grinding zone leads to collisions between grinding element, wall and material being ground and thus to the comminution of the particles. Due to the enormous surface enlargement and the high air flow rates, this principle is basically also suited for drying. This effect can be significantly intensified by using the available waste heat. The former test results show that the operating principle of the vortex mill, originally designed for grinding food or inorganic substances, can also be used successfully for grinding solid substitute fuels (Figure 4). Depending on the grinding resistance, reduction rates of up to >100:1 are possible. Surprinsingly, tests on different substitute fuel meals performed at the IEVB of the TU Clausthal showed ignition and Figure 4.First tests: These resulted in mixtures of a fine-particle and woolly fluffy substitute fuel meal. 500 400 ';;; E ';:. 300 .. 'ii "0 c g 200. c !!F • AF meal .s· - IT ,68() ·c > AF meal .7" · IT=711 •c _•_ Ltgmle du~t • RW£• IT:620 •c • Coal dust . £1 (('rrejon" lla760 •c L o+-------~--~L+~~~~------~----_, '500 600 700 BOO 900 1000 Temperature [ Cl Figure 5. Ignition delay as a function of the ignition temperature of solid substitute fuels compared to hard coal and lignite. conversion speeds more known to be associated with hard coal or lignite dusts (Figure 5) . The ignition temperatures (TZ) of the tested substitute fuel meals range between 680°C and 71 1 °C and thus, are located in the transition zone between lignite dust at TZ 620 °C and hard coal dust at TZ 760 °C. Thus, the substitute fuel meals show an ignition delay less than the one of hard coal dust and insignificantly more than lignite dust. This process is perfectly applicable to particles, which are three-dimensional , hard or brittle and so far have led to problems in kiln charge material. They are comminuted quickly and effectively, whereby thin two– dimensional flat particles are barely subjected to a comminution, which is also not required with regard to the burnout behaviour. While grain sizes reduce 100 per cent to < 3 mm in size and water content, the chemical properties of the substitute fuel components remain unchanged. The physical properties of the substitute fuel meal with regard to its flow and ignition behaviour are now comparable to lignite dust. Optimised clinkering zone firing system at the rotary kiln If substitute fuels are to be fed via the so-called clinkering zone burner, they must be further comminuted to dmax. 25 mm with heavy media removed for an improved burnout. In order to react appropriately to different fuel properties, also the burner must be designed according to the requirements. The clinkering zone burner POLFLAME VN proves to be especially suitable. It can be designed to burn primary fuels as well as refuse-derived fuels 42